Fixed "Address already in use" from liveserver.

Our WSGIServer rewrapped the socket errors from server_bind into
WSGIServerExceptions, which is used later on to provide nicer
error messages in runserver and used by the liveserver to see if
the port is already in use. But wrapping server_bind isn't enough since
it only binds to the socket, socket.listen (which is called from
server_activate) could also raise "Address already in use".

Instead of overriding server_activate too I chose to just catch socket
errors, which seems to make more sense anyways and should be more robust
against changes in wsgiref.